rownum in sql

Galaxy Glossary

What is the ROWNUM clause in SQL, and how is it used?

ROWNUM in SQL is a pseudo-column that assigns a unique row number to each row in a result set, ordered by the query's default order. It's crucial for retrieving specific rows based on their position in the result set. It's often used for pagination and selecting top N rows.

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Description

ROWNUM is a pseudo-column, not a regular column, meaning it's not stored in the table. It's assigned dynamically by the database system during query execution. The order of the rows in the result set is determined by the query's ORDER BY clause (if present). If no ORDER BY clause is specified, the order is arbitrary. Crucially, ROWNUM is assigned *before* any filtering or aggregation occurs. This means that if you filter the results, the row numbers will still reflect the original order. ROWNUM is particularly useful for tasks like retrieving the top 10 performers, or for implementing pagination in applications. It's important to note that ROWNUM is not guaranteed to be unique across multiple queries on the same table. Each query gets its own independent numbering.

Why rownum in sql is important

ROWNUM is a powerful tool for controlling the output of SQL queries. It allows developers to extract specific rows from a result set based on their position, which is essential for tasks like pagination, selecting top N records, and implementing complex data retrieval logic.

Example Usage

```sql SELECT employee_id, employee_name, salary FROM employees WHERE department_id = 10 ORDER BY salary DESC FETCH FIRST 5 ROWS ONLY; --Alternative using ROWNUM SELECT employee_id, employee_name, salary FROM ( SELECT employee_id, employee_name, salary, ROW_NUMBER() OVER (ORDER BY salary DESC) as rn FROM employees WHERE department_id = 10 ) ranked_employees WHERE rn <= 5; ```

Common Mistakes

Forgetting to use ORDER BY with ROWNUM, leading to unpredictable results.
Misunderstanding that ROWNUM is a regular column, leading to errors in queries.
Trying to use ROWNUM with aggregate functions like SUM or AVG, which will not work as expected.
Assuming ROWNUM is globally unique across multiple queries.

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